Tests under irradiation of optical fibers and cables devoted to corium monitoring in case of severe accident in a Nuclear Power Plant

Abstract

The DISCOMS project, which stands for “DIstributed Sensing for COrium Monitoring and Safety”, considers the potential of distributed sensing technologies, based on remote instrumentations and Optical Fiber Sensing cables embedded into the concrete floor under the reactor vessel, to monitor the status of this third barrier of confinement. This paper focuses on the selection and testing of singlemode (SM) optical fibers with limited RIA (Radiation Induced Attenuation) to be compliant with remote distributed instruments optical budgets, the ionizing radiation doses to sustain, and their reduction provided by the concrete basemat shielding. The tests aimed at exposing these fibers and the corresponding sensitive optical cables, to the irradiation doses expected during the normal operation of the reactor (up to 60 years for the European Pressurized Reactor), followed by a severe accident. Several gamma and mixed (neutron-gamma) irradiations were performed at CEA Saclay facilities: POSÉÏDON irradiator and ISIS reactor, up to a gamma cumulated dose of about 2 MGy and fast neutron fluence (E > 1 MeV) of 6 x 1015 n/cm2. The first gamma test permitted to assess the RIA at various optical wavelengths, and to select three radiation tolerant singlemode fibers (RIA < 5 dB/100 m, at 1550 nm operating wavelength). The second one was performed on voluminous strands of sensitive cables encapsulating the selected optical fibers, up to approximately the same accumulated dose, at two temperatures: 30°C and 80°C. A significant increase of the RIA, without any saturation tendency, appeared for fibers inserted into cables, correlated with the increase of the hydroxyl attenuation peak at 1380 nm. Molecular hydrogen generated by the radiolysis of compounds of the cable is at the origin of this phenomenon. A third gamma irradiation run permitted to measure the radiolytic hydrogen production yield of some compounds of a dedicated temperature cable sample. The efficiency of a carbon coating layer over the silica cladding, acting as a barrier against hydrogen diffusion, was also successfully confirmed. Finally, the efficiency of this carbon coating layer has also been tested under neutron irradiation, then qualified as a protection barrier against hydrogen diffusion in the optical fiber cores.

Remote monitoring of Molten Core-Concrete Interaction experiment with Optical Fibre Sensors & perspectives to improve nuclear safety – DISCOMS project

“…Additional telecom OTDR can be used for cable (optical fibre) length detection, along with reliable Fresnel's reflexion peak detection algorithms (resilient to optical losses). Strain sensing and security cables, with embedded radiation resistant optical fibres with a protective carbon coating layer [10], should be used for respectively early detection of corium vicinity (from 400°C in a safety role) and monitoring up to at least 750°C.…”

“…In parallel, several commercially available radiations resistant singlemode optical fibres were screen-tested, first as standalone fibres, then in sensing cables in order to determine the RIA (Radiation Induced Attenuation) generated by the cable compounds (polymers) in the optical fibre core. The irradiation tests performed within the industrial gamma-irradiator POSÉÏDON, and the research nuclear reactor ISIS, both located at CEA Saclay, have permitted to select rad-hard optical fibres that exhibit a maximum RIA < 50 dB/km at both 1310 nm and 1550 nm operating wavelengths, for cumulated gamma-dose close to 2 MGy [10].…”

“…It has also been proved that under irradiations, the cable polymer compounds release molecular hydrogen H2, which migrates towards the optical fibre core, and is finally responsible for significant additional RIA, without highlighting any tendency to saturation. One solution, qualified during this project, is to use an optical fibre with a protective carbon layer at its surface, thus acting as a barrier against hydrogen [10].…”

“…Combined with experimental RIA results versus cumulated dose, the modelling assesses that it makes sense to embed sections of 100 m of optical fibre(s) sensing cables at several depths in the concrete floor reactor pit. Minimum embedment depth of 30 cm and 5 cm below the top surface, for Gen II and Gen III respectively, will ensure correct measurements of cable lengths, strains & temperature profiles, as well as temperature changes from a reference trace [10].…”

“…It has already been assessed during this project that the best commercial radiation resistant fibres, embedded into their sensing cables, can sustain the expected radiation levels, without exceeding the maximum allowed RIA[10].…”